David Davis's resignation as Brexit Secretary has been hailed by fellow Eurosceptic Conservatives - and seized upon by opposition MPs as an indictment of Theresa May's Government.
Mr Davis quit on Sunday night in protest over Mrs May's approach to Brexit, days after signing up to a Cabinet plan agreed at Chequers.
Arch-Brexiteer Jacob Rees-Mogg said the "very important" departure would "give reassurance to backbench Conservative MPs who want a proper Brexit that this was being delivered".
"After the concerns over the Chequers summit, his departure raises even more questions over what came out of that meeting," he told BBC Radio 5 live.
Mr Rees-Mogg, who leads the influential European Research Group of leave-supporting MPs, said the PM's red lines were "rubbed out" at Chequers where attendees had been "steam-rollered" into supporting the PM's policies.
He said: "These proposals will have to come to the House of Commons in legislation and the question is, will they command support from Conservative MPs?
"And I think without David Davis there, without his imprimatur, it will be very difficult for them to get the support of Conservative MPs and therefore the Prime Minister would be well advised to reconsider them."
Mr Davis's exit is likely to embolden Brexiteer backbenchers with concerns about Mrs May's leadership.
Senior Conservative Bernard Jenkin said Mr Davis had been left in a "completely impossible" position.
Asked if Brexiteers needed to put the PM's future to a vote of the Conservative party, he replied "it may well come that".
He told the Today programme: "If the Prime Minister thinks she has consent and support from every member of her Cabinet she is deluding herself, as we have just seen."
He added: "There's been a massive haemorrhage of trust over the last few days because in all my meetings with the Prime Minister I never expected this to be the result.
Fellow Eurosceptic Tory MP Andrea Jenkyns, who resigned from her role as a ministerial aide over Brexit in March, said Mr Davis's departure was "fantastic news".
Speaking to Today, Ms Jenkyns said Mrs May's time was up as Prime Minister and she wanted a Brexiteer to lead the country.
"I think Theresa May's premiership is over," she said.
"I want one who passionately believes in Brexit and will provide true leadership and a positive post-Brexit vision for our country," she said.
Opposition MPs were quick to raise the prospect of a general election as a result of the major blow for the Prime Minister.
Labour leader Jeremy Corbyn said: "David Davis resigning at such a crucial time shows @Theresa_May has no authority left and is incapable of delivering Brexit.
"With her Government in chaos, if she clings on, it's clear she's more interested in hanging on for her own sake than serving the people of our country."
Shadow chancellor John McDonnell tweeted: "With a Prime Minister incapable of holding her ministerial team together & with such instability in government it's impossible to see how EU leaders could take Theresa May seriously in the next round of negotiations.
"It's time for her & her party to put country before party & go."
Seema Malhotra, who sits on the Commons Brexit Select Committee, tweeted: "Will there be a domino effect?
"It's now not inconceivable that May is gone within days or weeks, the Tories are plunged into disarray and a general election called."
Shadow education secretary Angela Rayner tweeted: "David Davis resignation now shows the Tory party is in complete disarray and not governing in the national interest.
"This nonsense cannot continue, even the Tory spin doctors cannot surely spin a positive message out of this debacle, it's time for the Tories to go #ToryFail"
Shadow foreign secretary Emily Thornberry tweeted: "Government resignations since autumn: 1 November 2017 - Fallon 8 November 2017 - Patel 20 December 2017 - Green 29 April 2018 - Rudd 8 January 2018 - Greening 8 July 2018 - Davis There have been six resignations in 249 days. That's one every six weeks."
The Liberal Democrats, who back a second referendum on Brexit, said on Twitter: "The resignation of David Davis is yet more evidence of the chaos of this Tory Brexit. You deserve the final say on this shambolic Brexit with the chance to stay in the EU."
Scotland's First Minister Nicola Sturgeon tweeted: "The #chequers unity didn't last long. This UK government is in utter chaos and ebbing authority by the day. What a shambles."
Ian Blackford, the SNP's Commons leader, tweeted: "It took the Tories two years to come up with a proposal and two days for it to fall apart!"
Mr Davis's departure was met with dismay from business leaders who had called for greater certainty over the Brexit process.
Carolyn Fairbairn, director-general of the CBI, said there had been "real rays of light" in the Cabinet agreement on Friday and described the resignation as a "blow".
She told Today: "This is a blow. One of the things that business welcomed on Friday was finally Cabinet unity.
"That inability to take decisions over several months had become a huge challenge in terms of uncertainty."